Commercial ceria-gadolinia powders were used to obtain bulk CGO samples, by sintering at 900 to 1500 degrees C with Co nitrate additions, or at temperatures in the range 1500-1600 degrees C, without Co. These samples were characterized by impedance spectroscopy in air, at temperatures in the range 150-600 degrees C, to distinguish the bulk and grain boundary behaviour. Addition of Co nitrate allows densification at lower temperatures and plays significant effects on both microstructural contributions of impedance spectra, enhancing the bulk and grain boundary conductivities and lowering their activation energy. Typical values of activation energy of bulk conductivity vary from 0.77 to 0.94 eV, and the activation energy of grain boundary conductivity were in the range 0.96 - 1.05 eV The effects of sintering additive are spoilt on raising the sintering temperature, due to depletion of Co content in grain boundaries of samples fired at high temperatures. These observations indicate that grain boundary behaviour may be determined by segregation of Co and/or Gd and the corresponding space charge layers, at least for materials prepared from high purity precursor powders. The p-type electronic conductivity is also enhanced for samples fired at relatively low temperatures with addition of Co, thus indicating that significant changes in defect chemistry occur. (c) 2005 Elsevier B.V. All rights reserved.